Reprint of Very low-grade metamorphism of the upper Permian Linxi Formation from the southern Da Xing'an Mountains, NE China: Constraints from clay mineral genesis

摘要

The metamorphic pressure-temperature conditions and the timing of metamorphism were determined for the upper Permian Linxi Formation, southern Da Xing'an Mountains, NE China, based on the genetic characteristics of clay minerals and K-Ar dating of illite from pelitic rocks. Detrital minerals are dominated by quartz and plagioclase, and clay minerals consist mainly of illite, chlorite, and mixed-layer chlorite/smectite. The XRD analytical results indicate that illite crystallinities are in the range of 0.25-0.57 (average value, 035), that illite polytypism is dominated by 2M(1) and minor 2M(1) + 1M(d) and 1M(d) types, and the illite b dimension is in the range of 8.9982-9.0312 angstrom (average, 9.0174 angstrom). These characteristics indicate that the strata containing the illite experienced very low-grade metamorphism. The temperatures estimated by vitrinite reflectance thermometry are in the range of 202 degrees C-293 degrees C (average, 245 degrees C), indicating that the pelitic rocks of the upper Permian Linxi Formation experienced anchizone-grade metamorphism. The illite grains used for dating are <0.15 mu m in size, which is similar to that of autogenic illite grains. The K-Ar age of the illite is 127.64 +/- 2.12 Ma, which is interpreted to represent the timing of the very low-grade metamorphism. (C) 2015 Elsevier B.V. All rights reserved.